commit 5aabd064f71bbaa38ea70ff01c28120e3f5e863f
Author: Joel Pittet <joel@pittet.ca>
Date:   Wed May 27 22:44:55 2015 -0700

    remove level

diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index 25359b1..a0cb290 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -501,9 +501,42 @@ public function getActiveTrailIds($bid, $link) {
   /**
    * {@inheritdoc}
    */
-  public function build(array $tree, $level = 0) {
+  public function build(array $tree) {
+    $items = $this->buildItems($tree);
+
     $build = [];
-    $items = [];
+
+    if ($items) {
+      // Make sure drupal_render() does not re-order the links.
+      $build['#sorted'] = TRUE;
+      // Get the book id from the last link.
+      $item = end($items);
+      // Add the theme wrapper for outer markup.
+      // Allow menu-specific theme overrides.
+      $build['#theme'] = 'book_tree__book_toc_' . $item['link']['bid'];
+      $build['#items'] = $items;
+      // Set cache tag.
+      $build['#cache']['tags'][] = 'config:system.book.' . $item['link']['bid'];
+    }
+
+    return $build;
+  }
+
+  /**
+   * Builds the #items property for a book tree's renderable array.
+   *
+   * Helper function for ::build().
+   *
+   * @param array $tree
+   *   A data structure representing the tree..
+   *
+   * @return array
+   *   The value to use for the #items property of a renderable menu.
+   *
+   * @throws \DomainException
+   */
+  protected function buildItems(array $tree) {
+    $items = array();
 
     foreach ($tree as $data) {
       $class = ['menu-item'];
@@ -541,24 +574,7 @@ public function build(array $tree, $level = 0) {
       $items[$data['link']['nid']] = $element;
     }
 
-    if (!$items) {
-      return [];
-    }
-    elseif ($level == 0) {
-      $build = [];
-      // Make sure drupal_render() does not re-order the links.
-      $build['#sorted'] = TRUE;
-      // Add the theme wrapper for outer markup.
-      // Allow menu-specific theme overrides.
-      $build['#theme'] = 'book_tree__book_toc_' . $data['link']['bid'];
-      $build['#items'] = $items;
-      // Set cache tag.
-      $build['#cache']['tags'][] = 'config:system.book.' . $data['link']['bid'];
-      return $build;
-    }
-    else {
-      return $items;
-    }
+    return $items;
   }
 
   /**